The present invention relates generally to the charging of lithium sulfur batteries, and more particularly to systems and methods for accurately determining the state of charge and the relative age of lithium sulfur batteries.
The ability to discern how much energy is stored in a rechargeable battery of a portable consumer electronic device, such as a cellular telephone or laptop computer, is a feature that is highly valued by the user of the device. Therefore, common device systems, such as those using lithium-ion, nickel metal hydride, or nickel-cadmium rechargeable batteries, incorporate some technique to gauge the amount of energy or charge presently stored in the battery cell. One common approach is to determine the state of charge of the battery based upon the measured open-circuit voltage for that battery using look-up tables. See, for example, U.S. Pat. No. 6,789,026 to Barsoukov et al. and U.S. Pat. No. 6,774,636 to Guiheen et al., each of which is hereby incorporated by reference herein in its entirety.
The state of charge (“SOC”) of a battery is the presently stored charge expressed as a fraction of the maximum charge that can be stored in the battery. The SOC of a battery is very useful information in that its user may know how charged the battery is relative to the maximum charge or capacity of the battery during its current charge/discharge cycle. However, the maximum capacity of a battery degrades with the “age” of the battery (i.e., the number of charge/discharge cycles to which the battery has been subjected, and not the actual amount of time that the battery has existed). The above-described conventional open-circuit voltage-based algorithms do not use stored look-up tables that adequately represent the characteristics of the battery as it ages to determine its state of charge.
Lithium sulfur batteries have gained favor in recent years due to their light weight and high energy density. The use of lithium anodes (e.g., lithium foil or vacuum deposited lithium of either pure lithium or lithium alloyed with tin or aluminum, with or without an integral current collector or various lithium intercalation compounds, such as graphites, cokes, and tin oxide, etc.) provides an opportunity to construct lithium sulfur battery cells that are lighter in weight and have a higher energy density than cells such as lithium-ion, nickel metal hydride, or nickel-cadmium cells. These features are highly desirable for batteries in portable electronic devices.
Lithium sulfur battery designs are particularly suitable for portable electronic devices because of their light weight and their high surface area, which allows high rate capability as well as reduced current density on charging. Several types of cathode materials for the manufacture of lithium batteries are known, including cathode materials having sulfur-sulfur bonds, wherein high energy capacity and rechargeability are achieved from the electrochemical cleavage (via reduction) and reformation (via oxidation) of the sulfur-sulfur bonds. Sulfur containing cathode materials, having sulfur-sulfur bonds, for use in electrochemical cells having a lithium anode, such as described above, may include elemental sulfur, organo-sulfur compounds, various polysulfides, or carbon-sulfur compositions.
Accordingly, it would be desirable to provide systems and methods for accurately determining the state of charge of lithium sulfur battery cells, and for accurately determining the age of battery cells.